使用onClick方法启动另一个活动时出错

Question

I am using the same onClick method for two buttons to launch an activity, one of them to go to my Settings page and the other to go a Scanner page. The settings button is working fine but the scanner one is not. I just can't seem to find the problem. I added the activity already in my manifest. Still could not work.

MainPage.java

package mapp.com.sg.receiptscanner;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

public class MainPage extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_page);

    Button ScannerBtn = (Button)findViewById(R.id.ScannerBtn);
}

public void onClick(View view) {
    switch (view.getId()) {
        case R.id.SettingsBtn:
            Intent i = new Intent(this, Settings.class);
            startActivity(i);
            break;
        case R.id.ScannerBtn:
            Intent x = new Intent(this, Scanner.class);
            startActivity(x);
            break;


    }
}
}

Scanner.java

package mapp.com.sg.receiptscanner;

import android.content.Intent;
import android.graphics.Bitmap;
import android.os.Bundle;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;



public class Scanner extends AppCompatActivity {

ImageView imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_scanner);

    Button btnCamera = (Button)findViewById(R.id.btnCamera);
    imageView = (ImageView)findViewById(R.id.imageView);

    btnCamera.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
            startActivityForResult(intent,0);
        }
    });
}

protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    Bitmap bitmap = (Bitmap) data.getExtras().get("data");
    imageView.setImageBitmap(bitmap);
}

}

Logcat

08-13 12:11:47.996 2785-2785/mapp.com.sg.receiptscanner E/AndroidRuntime: FATAL EXCEPTION: main
                                                                      Process: mapp.com.sg.receiptscanner, PID: 2785
                                                                      java.lang.RuntimeException: Unable to start activity ComponentInfo{mapp.com.sg.receiptscanner/mapp.com.sg.receiptscanner.Scanner}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                                                                          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                                                                          at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                                          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                                                                          at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                          at android.os.Looper.loop(Looper.java:154)
                                                                          at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                                          at java.lang.reflect.Method.invoke(Native Method)
                                                                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
                                                                       Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                          at mapp.com.sg.receiptscanner.Scanner.onCreate(Scanner.java:25)
                                                                          at android.app.Activity.performCreate(Activity.java:6662)
                                                                          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                                                                          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
                                                                          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                                                                          at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                                                                          at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                          at android.os.Looper.loop(Looper.java:154) 
                                                                          at android.app.ActivityThread.main(ActivityThread.java:6077) 
                                                                          at java.lang.reflect.Method.invoke(Native Method) 
                                                                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) 

activity_scanner.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="16dp"
android:paddingLeft="16dp"
android:paddingRight="16dp"
android:paddingTop="16dp"
tools:context="mapp.com.sg.receiptscanner.Scanner">

<ImageView
    android:layout_width="match_parent"
    android:layout_height="fill_parent"
    android:layout_weight="9"
    android:id="@+id/imageView"/>

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_weight="1"
    android:background="@android:color/holo_blue_light"
    android:textColor="@android:color/white"
    android:text="Open Camera"/>

activity_main_page.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="mapp.com.sg.receiptscanner.MainPage">

<Button
    android:id="@+id/ScannerBtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="38dp"
    android:text="Scanner"
    android:onClick="onClick"
    tools:layout_editor_absoluteX="130dp"
    tools:layout_editor_absoluteY="47dp" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Receipts"
    tools:layout_editor_absoluteX="148dp"
    tools:layout_editor_absoluteY="140dp"
    android:layout_below="@+id/ScannerBtn"
    android:layout_alignStart="@+id/ScannerBtn"
    android:layout_marginTop="47dp" />

<Button
    android:id="@+id/SettingsBtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Settings"
    android:onClick="onClick"
    tools:layout_editor_absoluteX="148dp"
    tools:layout_editor_absoluteY="231dp"
    android:layout_centerVertical="true"
    android:layout_alignStart="@+id/button2" />

Answer 1

In class Scanner, btnCamera is null. You should add android:id="@+id/btnCamera" to button in activity_scanner.xml

Answer 2

您的activity_scanner.xml中没有为相机按钮定义ID。